1. Field of the Invention
The present invention relates to a hydraulic driving apparatus provided in a working machine, such as a crane, to move a load, such as a suspended load.
2. Description of the Background Art
As a hydraulic driving apparatus provided in a working machine, there is known a type equipped with a hydraulic actuator, as described, for example, in JP 2000-310201 A, which will be called “Patent Document 1” below. In this type, the hydraulic actuator can be operated to move a load in the same direction as a self-weight falling direction which is a direction along which the load falls by its self-weight. Patent Document 1 discloses a lowering hydraulic circuit for operating a hydraulic motor serving as the hydraulic actuator, to move a suspended load in a lowering direction.
In the above working hydraulic driving apparatus, cavitation is likely to occur during a lowering drive mode in which the hydraulic motor is driven (operated) in a rotational direction corresponding to the lowering direction. Specifically, during the lowering drive mode, the self-weight of the suspended load increases the rotational speed of the hydraulic motor, which may cause a flow rate adsorbed by the hydraulic motor to be greater than a flow rate of hydraulic fluid supplied from a hydraulic pump to the hydraulic motor. This may lower a pressure of a meter-in fluid passage which is a fluid passage on a hydraulic fluid supply side of the hydraulic motor, namely a meter-in pressure, to generate cavitation in the meter-in fluid passage. The cavitation possibly steals a braking force from the hydraulic motor and possibly causes falling of the suspended load.
To suppress cavitation thus generated in a meter-in fluid passage, Patent Document 1 discloses an external pilot-operated counterbalance valve (hereinafter referred to simply as “counterbalance valve”) provided in a meter-out fluid passage, that is, a fluid passage on a hydraulic fluid discharge side of the hydraulic motor (counterbalance valve 11 in FIG. 1 of Patent Document 1). The counterbalance valve is applied with a meter-in pressure as a pilot pressure and applied with a set pressure P1 thereof by means of a spring or the like. The counterbalance valve has a variable valve opening degree, which is increased when the meter-in pressure becomes greater than the set pressure P1 while reduced when the meter-in pressure becomes lower than the set pressure P1. The counterbalance valve narrows down the meter-out fluid passage when the valve opening degree is reduced, thus producing a braking force in the hydraulic motor to decelerate the hydraulic motor and thereby suppress the flow rate adsorbed by the hydraulic motor. The counterbalance valve thus keeps the meter-in pressure to the set pressure P1 or below, thereby suppressing the cavitation in the meter-in fluid passage.
FIG. 16 shows a conventional working hydraulic driving apparatus 701, which comprises an external pilot-operated counterbalance valve 784. The counterbalance valve 784 has a measurement point on the meter-in fluid passage 30 and a control point is located on the meter-out fluid passage 40; these forms an unstable control system which is not co-located under the control theory.
This unstable control system is likely to cause hunting in rotational speed of the hydraulic motor 13. For example, when a manipulation lever 16 shown in FIG. 16 is manually operated from a neutral position to a lowering position thereof at a time T0, an amount of hydraulic fluid to be supplied to the hydraulic motor 13 through a direction selector valve 20 is increased according to the manual operation, thus increasing a meter-in pressure of the meter-in fluid passage 30. The counterbalance valve 784, detecting the increase in the meter-in pressure, is operated in a valve-opening direction; however, the difference between the measurement point and the control point of the counterbalance valve 784 intends to cause a time lag from a change in the meter-in pressure to a real movement of the valve spool of the counterbalance valve 784. This time lag repetitively increases and reduces the valve opening degree of the counterbalance valve 784 as shown in FIG. 17A, thereby oscillating the meter-in pressure as shown in FIG. 17B. This results in a possibility of an oscillation in rotational speed of the hydraulic motor 13 (see FIG. 16), that is, a hunting.
As means for suppressing such hunting, it is conceivable to provide an orifice 786 shown in FIG. 16 in a pilot line 785 for the counterbalance valve 784. The orifice 786 makes the counterbalance valve 784 be gradually opened according to an increase in the meter-in pressure. In other words, the orifice 786 provides attenuation to a movement of the counterbalance valve 784 in a direction from a closed state to an opened state, thereby making a response of the valve 784 slow.
The orifice 786, however, generates flow resistance due to the throttle of the meter-out fluid passage by the counterbalance valve 784, until the counterbalance valve 784 will have reached an adequate valve opening degree A1 as shown in FIG. 18A, thereby possibly generating an unnecessary boosted pressure in the meter-in fluid pressure 30, as indicated by the shadow area in FIG. 18, which possibly results in deteriorated fuel economy.
Patent Document 1 discloses a technique of providing a flow regulation valve in order to suppress the hunting. This flow regulation valve is operable to control a flow rate in the meter-in fluid passage so as to reduce the pressure difference between the meter-in fluid passage and the meter-out fluid passage. This however causes a problem that an operating speed (lowering speed) of the hydraulic motor is largely varied depending on a mass of the load. The reason is as follows.
When a hydraulic driving apparatus is operated in a direction to move a load downward, i.e., in a lowering direction, a holding pressure appropriate for a mass of the load is generally produced in a meter-out fluid passage. This holding pressure becomes higher when the load is relatively heavy, than when the load is relatively light. The flow regulation valve disclosed in the Patent Document 1 opens more largely as the holding pressure becomes higher. This increases a flow rate in the meter-in fluid passage, that is, a meter-in flow rate, and operating speed of the hydraulic motor. Hence, the lowering speed becomes higher in the case of a relatively heavy load than in the case of a relatively light load. In other words, even if a manipulation lever is not changed, the operating speed of the hydraulic motor can be varied depending on a level of weight of the load. This deteriorates operability.
JP 10-267007 A, which will be called Patent Document 2 below, discloses a regeneration circuit, for example, shown in FIG. 5 of Patent Document 2. This regeneration circuit comprises an orifice provided in a meter-out fluid passage and a regeneration fluid passage for communicating an upstream side of the orifice and a meter-in fluid passage. The regeneration circuit allows a part of hydraulic fluid flowing through the meter-out fluid passage to be returned to the meter-in fluid passage through the regeneration fluid passage, thereby increasing respective operating speeds of a hydraulic actuator and an attachment adapted to be driven by the hydraulic actuator.
If the regeneration circuit disclosed in Patent Document 2 were applied to a circuit where a hydraulic motor is used to lower a suspended load as disclosed in the Patent Document 1, a flow rate in the regeneration fluid passage (regeneration flow rate) would be increased along with an increase in weight of the suspended load. This makes the lowering speed become higher as the suspended load becomes heavier, resulting in a loss of safety and operability. Moreover, the technique disclosed in the Patent Document 2, which does not compensate for a minimum pressure of the meter-in fluid passage, has a possibility of cavitation in the meter-in fluid passage, which may cause the hydraulic motor to stall.